1. Field of the Invention
The present invention relates to an array coil suitably used as the receiving RF coil of a magnetic resonance imaging apparatus. The present invention also relates to a magnetic resonance imaging apparatus provided with the array coil.
2. Description of the Related Art
Array coils are in wide use as receiving radio frequency (RF) coils of magnetic resonance imaging apparatuses (MRI apparatuses). An array coil is made by arranging a plurality of element coils.
Known array coils include a type provided with a large number of element coils and enabling a wide imaging region. When this type of array coil is put to use, the field of view (FOV) may be narrower than the sensitivity region of the entire array coil. In this case, the element coils are partially made effective in such a manner that the size of the actual sensitivity region corresponds to the size of the FOV.
Jpn. Pat. Appln. KOKAI Publication No. 4-212329 discloses a magnetic resonance imaging apparatus related to the above technology. In the magnetic resonance imaging apparatus of the publication, only the coil assemblies that are used for imaging are selected and made effective.
Where the sensitivity region required in accordance with the FOV (the sensitivity region will be hereinafter referred to as the “required sensitivity region”) is attained by part of the element coils, the element coils that are made effective are selected from all element coils of the array coil. Then, each of the required sensitivity regions has to overlap each of the sensitivity regions of the selected element coils (which will be hereinafter referred to as “individual sensitivity regions”). Depending upon the positional relationships between the FOV and each element coil, therefore, the number of element coils that have to be selected may be larger than the minimal number of element coils required for forming the actual sensitivity region having the same size as the FOV. Since, in this case, the sensitivity region is larger than the FOV, aliasing artifacts may occur.
As a method for reducing the number of selected element coils to a value approximately equal to the above-mentioned minimal number, it is thought to employ element coils whose width is decreased with respect to the arrangement direction of the element coils. It is also thought to widen the overlap portion between the adjacent element coils. However, the former method has problems in that the sensitivity is degraded in regions away from the array coil. Likewise, the latter method has problems in that the interference between the element coils is so intense that the SN ratio is degraded.